1. Field of the Invention
The present invention relates to material powder used for fabrication of silicon nitride ceramics products which are used for various types of structural materials such as automobile parts. More particularly, the invention relates to the surface characteristics of silicon nitride material powder essential to obtain a high-strength, high-reliability sintered body.
2. Description of the Prior Art
Silicon nitride ceramics are lightweight, superior in heat-, corrosion-, and wear-resistance, and well-balanced in strength and toughness, so that they are expected to be put into practical use in a wide range of applications such as engine parts of automobiles and gas turbine engines. However, the silicon nitride ceramics are brittle such that their mechanical characteristics are largely affected by various defects present in the interior of the sintered body. Among others, their strength characteristic is vulnerable to defects such as voids, impurities, or coarse particles and flaws. These parts have stresses concentrated thereon enough to make sources of fracture, causing the strength of the sintered body to lower to a substantial extent. As a result, silicon nitride ceramics have hitherto been lacking in reliability required to be a practical material.
A silicon nitride sintered body is obtained by firing and thereby solidifying a powder. In more detail, a material powder mixed with a sintering assistant or the like is compacted, and heated in a firing oven, whereby the powder particles are sintered With one another to form a polycrystalline body. Therefore, microstructure of a sintered body will be intensely affected by the characteristics of the material powder.
Conventionally, it has been assumed that the silicon nitride material is preferably provided by a powder that has a high .alpha.-fraction of a crystal phase, finer particles, and as small amounts of metal impurities as possible, for the purpose of implementing a sintered body which is homogeneous and has fewer defects. In view of this, material manufacturers have competitively developed material powders having high .alpha.-fractions, fine particles, and high degrees of purity. As a result, there has become commercially available high-quality materials of which the .alpha.-fraction is over 90%. The mean particle size is under 1 .mu.m. and the iron content is under 100 ppm. This has contributed to a substantial reduction in the generation of impurities end coarse particles contained in the sintered body. Thus, the frequency of occurrence of fractures due to these defects has been lowered by adjusting the sintering conditions.
Further, there have been proposed techniques for removing impurities other than metals present in the surface by surface-treating a produced silicon nitride powder, or for obtaining a high-performance sintered body by varying the surface properties. For example, Japanese Patent Publication No. HEI 4-65002 has disclosed a method for removing chlorine and/or fluorine from silicon nitride by bringing a silicon nitride powder containing chlorine and/or fluorine into contact with a gas containing chlorine and/or fluorine into contact with a gas containing water vapor at a temperature above 350.degree. C. and below 1100.degree. C. A claim of Japanese Patent Laid-Open Publication No. HEI 4-83758 has disclosed a method using a silicon nitride powder cleaned with a high-purity water. Other available methods include: a method, as disclosed in a claim of Japanese Patent Laid-Open Publication No. HEI 2-107509, for producing a silicon nitride powder with a total oxygen content lower than 1.8 wt %, a ratio of surface oxygen content to total oxygen content being more than 65%, and a fluorine content lower than 35 ppm, the method comprising a step of annealing Si.sub.e N.sub.4 powder with a total oxygen quantity of not more than 0.4 wt % in an oxygen-contained atmosphere at a temperature of 700.degree.-1200.degree. C. for 15-90 min.; a method, as disclosed in a claim of Japanese Patent Laid-Open Publication No. HEI 3-199167, for heat treating a silicon nitride powder with a BET specific surface area of 6 m.sup.2 /g or more and an oxygen content of 0.3-1.8 wt % in air or an oxidizing atmosphere at a temperature of 250.degree.-800.degree. C. so that the increment in the quantity of oxygen contained is 0.1 wt % or less for every 1 m.sup.2 surface area of the silicon nitride powder; a method, as disclosed in claim of Japanese Patent Laid-Open Publication No. HEI 2-172807, for treating a silicon nitride powder with a mixed acid of fluoric and nitric acids; and a method, as disclosed in a claim of Japanese Patent Publication No. 5-13083, wherein a slurry in which water has been added to silicon nitride powder is heated and, with the resultant temperature maintained at 35.degree.-100.degree. C., the silicon nitride powder is wet-crushed while the content of oxygen contained in the silicon nitride powder is increased at the same time.
As shown above, the technique for fabricating fine and high-purity silicon nitride powders has been advanced so far, and further, various types of powder surface treating techniques have been developed. Along with these aspects of advancement, the reliability of silicon nitride sintered bodies and therefore of silicon nitride ceramics products have been greatly improved.
However, silicon nitride ceramics are still insufficient in reliability when used as parts required to have high reliability, such as automobile parts. In other words, silicon nitride ceramics are still low in the fracture strength level and large in the variation of strength.
The present inventors have investigated the causes of the aforementioned problems and, as a result, have found that structure of the compact before sintering accounts for the fact that the resulting silicon nitride sintered body varies in strength and is low in reliability, even when a high-purity fine powder is used. More specifically, in fabricating ceramics, a compacting (or molding) process is necessary to change the powder into a shaped solid. In this process, the compacted body has clearances or voids present among the packed powder particles. These voids, if distributed uniformly in a very small size, would be completely eliminated in the subsequent firing process. However, if the distribution is nonuniform such that large size voids are maldistributed, they will not be eliminated but remain as defects in the sintered body. As a result, no matter how compact a sintered body with fine particles and without impurities is made; these slightly remaining voids would cause the fracture strength of ceramics to lower to a substantial extent.
In this way, conventional silicon nitride sintered bodies would allow the presence of such voids as will incur deterioration of fracture strength. This is due to coarse voids that are generated by the heterogeneous packing structure of powder in the compact as described above. To this point, many engineers have devoted themselves to improvement of this problem experimentally in their own fashion by repeatedly attempting a wide variety of compacting and sintering experiments.